1. Field of the Invention
This invention relates to a control device of a transmission for a vehicle in which shift is set by a hydraulic control system.
2. Description of the Prior Art
Conventionally, in a vehicle transmission, comprising a shift mechanism which effects shifting by the operation of hydraulic actuators and a hydraulic control device for supplying working oil to the hydraulic actuators or discharging it from same to control the shift of the shift mechanism when the supply of the working oil is changed over from one hydraulic actuator to the other to effect shifting, the working oil is supplied with line pressure regulated by hydraulic pressure generated by an oil pump, a throttle opening or the like.
For example, an auxiliary transmission of a four wheel drive transmission capable of selectively providing shift stages uses a planetary gear device for the shift mechanism. An input shaft of the auxiliary transmission is connected to a sun gear of the planetary gear device. An output shaft of the auxiliary transmission is connected to a carrier of the planetary gear device. A clutch for causing a sun gear to engage and disengage from the carrier and a brake for fixing and releasing a ring gear of the planetary gear device are provided to obtain the direct stage by engaging the clutch and releasing the brake and the deceleration stage by releasing the clutch and applying the brake. The change-over of engagement and disengagement of the clutch and brake is carried out by a low-high change-over valve which changes over working oil for a hydraulic servo of a frictional engagement device for the clutch and the brake. When in a vehicle having such a four wheel drive transmission, down shift from the direct stage to the deceleration stage is effected under a certain vehicle speed, the carrier is rotated with a certain speed since it is connected to the output shaft of the four wheel drive transmission. Load applied to the sun gear is reduced by releasing the clutch, the rotational speed of the sun gear is increased and that of the ring gear is reduced by the output of a main transmission. In such prior art constructions problems have been encountered in that when the engagement of the brake is delayed, the rotational speed of the sun gear is increased (during idling of engine) since the low load condition of the output shaft of the main transmission is continued, and when the engagement of the brake occurs too soon, a peak torque is produced in the shift torque fluctuation since the rotational speed of the ring gear is forcibly made zero. To overcome these problems, it is necessary to increase the rotational speed of the sun gear by the output of engine and to supply instantaneously working oil to the brake-engaging hydraulic servo for engaging the brake when the rotational speed of the ring gear is made zero. While the low-high change-over valve is changed over and the working oil in the clutch is exhausted to start the supply of working oil to the brake, the rotational speed of the ring gear will be reduced to zero with a certain delay after the supply of working oil to the brake-engaging hydraulic servo is started, so that hydraulic pressure has been adapted to be gradually supplied to the brake-engaging hydraulic servo in the single hydraulic pressure supply characteristics.
In the above-mentioned prior art, since the hydraulic pressure supplied to the hydraulic actuator in the shift is adapted to be gradually supplied, the working oil is supplied to the brake before the rotational speed of the ring gear is made zero in the shift from the direct stage to the deceleration stage, for example, so as to forcibly stop the ring gear whereby the peak torque is produced in the shift torque fluctuation Also, even after the rotational speed of the ring gear is made zero, the problem was encountered that the ring gear was rotated in the reverse direction and the engine was destroyed since oil of insufficient pressure was supplied to the brake.